Antenna system and net drift verification

ABSTRACT

System and method for in-operation calibration of phased array antenna system, involv-ing outputting first and second calibration signals on separate calibration nets through same switches of transmit and receive modules (TRM) and transmit and receive feed network branches for detecting whether system is operational. Allows moreover, identifi-cation of drift in specific calibration net and compensation therefore.

This application is a 371 of PCT/SE02/01522 filed Aug. 26, 2002.

FIELD OF THE INVENTION

The present invention generally relates to an antenna system networkarchitecture and a method for detecting and correcting drift in such anetwork. More particularly the present invention relates to an activephased array antenna system and a calibration method for such a system.

BACKGROUND OF THE INVENTION

In some antenna systems with steerable beams or directionalcapabilities, such as active phased array radar antennas or activetelecommunication base-station antennas, numerous antenna elements arearranged equidistantly in a plane whereby each element is adapted toemit and receive microwave signals.

In those systems, typically a microwave exciter and receive unit isprovided for generating and receiving a signal. The exciter and receiveunit is connected to a branch like transmission network, through whichsignals are emitted and received. The transmission network connects torespective transmit and receive modules, TRM, comprising electricallycontrollable phase shifters and amplifier stages through which theamplitude and phase delay of signals can be controlled. The transmit andreceive modules are connected to the antenna elements. Typically, dipoleelements may be used as antenna elements.

When signals are provided simultaneously to the plurality of elements, aplane wavefront parallel to the plane of the array is generated becauseof the in-phase interference of individual signals. When the phases ofsignals are incremented for each antenna element with regard to anadjacent element, a wavefront is propagating at a non-parallel anglewith regard to the plane of the elements, which angle is dependent onthe incremental phase delay. The elements may also be arranged atnon-equidistant intervals, but then the individual delays arecorrespondingly controlled to provide a plane wavefront. By arranging aplurality of elements on a plane and controlling the emission withregard to two directions, the resulting direction of the emitted beamand the sensitivity of the received signal may be controlled inthree-dimensional space.

One disadvantage associated with known active antenna systems is theamount of hardware required. A phased array antenna may for instancehave several thousand individually controllable antenna elements.

Moreover, it is important that all individual transmission paths are ofthe same or of known length to accomplish the desired beam-steeringcontrol over the desired bandwidth.

An important characteristic of an antenna system with high sensitivityis the directional properties as expressed by the level of theside-lobes compared to the level of the main lobe.

For instance for airborne radar systems, such as Airborne Early Warning(AEW) systems, the side-lobes must be so well attenuated that unwantedground and sea clutter can be efficiently suppressed. Low sidelobes arealso required in order to suppress signals from other emitters in theneighbourhood such as signals from active hostile jamming. The lowsidelobe level specification necessitates a tight control of theamplitude and phase of each transmit/receive module, TRM. Whentransmitting, the amplitudes of all TRMs have identical settings,whereas amplitude tapering is applied in receive mode. In air-cooledsystems, the phase and amplitude control must cope with the largetemperature variations that may prevail. This particularly applies toair-borne radar systems. For instance the feed and receive network maybe subject to thermal expansion/contraction, which alters the phase ofindividual signals. One example of AEW system has been shown in U.S.Pat. No. 4,779,097.

Generally, antenna systems are complex systems with many components,which require accurate control.

In a distributed transmission system, utilising microwave wave-guides,the transmit and receive modules account for a majority of the errorsthat are introduced. Careful design of these parts with respect tolong-term stability of performance, supply voltages, internal heatingand ambient temperature is necessary but often not sufficient.Therefore, a need has arisen as to be able to calibrate antenna systemsduring operation.

In FIG. 1, a known antenna system has been shown. The system comprisesan exciter/receiver unit ERU, a plurality of dipole antennas D1–Dk,respective couplers Q1–Qk being arranged adjacent the respective dipoleantennas, a feed and receive transmission network (R) connecting theexciter/receiver unit and a plurality of T/R modules TRM1–TRMk, anotherfeed network, AF, and a calibration network, C1.

The 1-k antenna elements may be evenly dispersed over a rectangularplane in a pattern of rows and columns.

In FIG. 1, a subset of the antenna elements, for instance D1–Dk,corresponding to a first row (or column) has been shown for simplicity.It should be understood that typically many more elements would form thefirst row and that subsequent elements up to element Dkk wouldcorrespond to additional rows.

The exciter/receiver unit, ERU, has a data bus XD, over which theexciter receiver unit controls the individual transmit and receivemodules TRM for obtaining the desired directional capabilities.

Each respective T/R module has a feed AF that leads to an antennaelement. The calibration network C constitutes a branch like structurewith equally long distance to each respective coupler Q1–Qk. Calibrationsignals are sent through a port X, of the ERU, returning through aselected transceive and receive module TRM and returning through thefirst feed network R back to the ERU over the transmission network, R.The phase and amplitude of the signal is compared to a fixed referencefor a given path. This procedure is completed for all transceive andreceive modules, TRM.

Prior art document U.S. Pat. No. 5,412,414 shows a similar phased arrayradar system providing in-operation calibration. The radar systemcomprises an exciter, a receiver, a transmit/receive transmissionnetwork, T/R modules and dipole elements. Respective directionalcalibration couplers are provided adjacent the dipole elements fortransferring signals through these to/from a calibration network whichis different from the transmit/receive network. By issuing transmittingcalibration signals from the exciter and leading signals through thetransmit/receive transmission network and through individual T/R modulesto couplers adjacent selected dipole elements, and return through thecalibration network, variations in the transmit/receive network andassociated components can be analysed. Likewise, by issuing receivingcalibration signals from the exciter and leading signals through hecalibration network to couplers adjacent selected dipole elements, andreturn through T/R module and the transmit/receive network, variationsin the transmit/receive network and associated components can beanalysed. One drawback with the above system is that an initialcalibration, using external measurement equipment, seems to be requiredbefore in-operation calibrations can be carried out.

U.S. Pat. No. 5,874,915 shows an AEW phased array system having aplurality of selector switches for coupling a respective low noisereceive amplifier or transmit amplifier to one of three antenna elementsin a respective column of the antenna array.

SUMMARY OF THE INVENTION

It is a first object of the present invention to set forth an antennasystem, which allows verification of error free operation while thesystem operates or in direct connection with operation.

It is a second object of the invention to set forth an antenna system inwhich calibration nets or branches through a calibration net can becompared.

It is a third object of the invention to achieve a calibration networkstructure that can be cost effectively produced.

It is a fourth object of the invention to enable the drifts in panelscaused by e. g. hot or cool spots to be detected.

It is a fifth object to accomplish extensive calibration possibilitiesin an antenna system of less extensive complexity with a reduced numberof components.

It is a further object to detect drift in a calibration net for anantenna system.

It is another object to establish which calibration net is drifting.

It is a still further object to calibrate a calibration network.

Further advantages will appear from the following detailed descriptionof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a known phased array antenna system comprising acalibration network,

FIG. 2 shows a phased array antenna system according to a firstembodiment of the invention,

FIG. 3 shows a transmit and receive module according to a firstembodiment of the invention,

FIG. 4 shows the antenna system according to a first embodiment of theinvention, comprising phased array antenna panels as seen from above,

FIG. 5 is a schematic illustration of arranging calibration nets of aphased array antenna panel,

FIG. 6 is a schematic illustration of a second embodiment of arranging acalibration net of a phased array antenna panel according to theinvention,

FIG. 7 is a schematic illustration of a third embodiment of arranging acalibration net of a phased array antenna panel according to theinvention,

FIG. 8 shows a first step of a preferred calibration routine accordingto the invention,

FIG. 9 shows a second step of a preferred calibration routine accordingto the invention

FIG. 10 shows a third step of a preferred calibration routine accordingto the invention,

FIG. 11 shows a second embodiment of a phased array antenna systemaccording to the invention,

FIG. 12 shows a third embodiment of a phased array antenna systemaccording to the invention, and

FIG. 13 shows an embodiment of a transceive and receive module accordingto the third embodiment of the antenna system according to theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 2, a first preferred embodiment of an antenna system accordingto the invention has been shown. The system comprises a main unit MUcomprising, an exciter/receiver unit, ERU, a beamforming unit BFU, acalibration unit CU, a plurality of antenna elements (D), a feed andreceive transmission network, R, connecting the exciter/receiver unitand a plurality of T/R modules, TRM1–TRMk, antenna feed branchesAF1–AFn, connecting T/R modules and antenna elements. Each respectiveT/R module has a plurality of ports P1–Pn, whereby each port leads to anantenna element D11–Dnhv.

A plurality of couplers Q111–Qnhv is provided, such that for eachantenna element D there is preferably arranged a coupler Q adjacent anantenna element. As appears from the figure, for instance coupler Q211is arranged next to the antenna element D211. Preferably, the respectivecoupler is arranged close to the antenna element and adjacent a segmentof the antenna feed AF2 for that particular element. A plurality ofcalibration networks are provided, C1–Cj, and each calibration networkcouples to a subset of the couplers Q111–Qnhv. Each calibration netbranches off from a port, X1–Xj, on the calibration unit, CU, to variouscouplers belonging to a subset of antennas to be served. Hence, eachcalibration network is separate and connected to a designated port onthe calibration unit, CU. The separate calibration networks are notconnected to one another. By way of example, if a signal is issued onport X1 of CU, the signal is lead to calibration network C1 and tocouplers Q111, Q112, Qn12, Q1hv and coupled to respective antenna feedsto corresponding ports on the various T/R modules. Likewise, a signal onport P1 of TRM1 will be transferred to coupler Q111, calibration networkC1 and port X1 of calibration unit CU.

Advantageously, the couplers are formed as strips arranged at a pointadjacent a given antenna feed and close to the antenna element inquestion. However; the couplers may also be electrically connected overa suitable impedance or waveguide to the antenna element. The couplermay be realised in stripline technology as is well known in the art.

The calibration nets and the feed and receive net are advantageouslyformed as tree structures with a number of branches. However, other gridstructures and non-branched structures are also possible.

In FIG. 3, a T/R module according to the invention has been shown. TheT/R module TRM is capable of transmitting and receiving signals to andfrom the respective antennas, the calibration network (C1–Cj) and thetransmission network, R, according to suitable control from the mainunit MU over the databus XD.

The T/R module comprises a phase shift and attenuation module, PSAM, apower amplifier module, PAM, a logic bias board, LBB, a low noiseamplifier, LNA, and a switch module, SM. The logic bias board, LBB,controls the individual functions of the above mentioned modules, suchthat the phase and amplitude of both transmitted and received signalsare controlled according to a desired directivity pattern in aconventional fashion. The logic bias board, LBB, moreover controls theswitch module, SM, to select radio frequency signals to or from the XRport of the ERU to one of the various ports P1 to Pn in a manner, whichshall be explained later.

In transmit mode, the antenna system controls the beam forming in aconventional way. A microwave signal is emitted on the transmissionnetwork R. This signal is received at the various T/R modules TRMsubstantially synchronously. In each T/R module (1-k) for eachconnection (1-n), the input signal is processed individually, such thatrespective signals to respective antenna elements (1-h·v) have therequired phase and amplitude properties for obtaining the desired beamform. For this purpose, corresponding signals are sent over port XD tologic bias board, LBB, which controls the phase shift and attenuationmodule, PSAM, and the power amplifier module, PAM.

In receive mode, the antenna system accomplishes the required focussingin a conventional way by controlling the phase and amplitude of incomingsignals from low noise amplifier LNA in each respective T/R module. Forthis purpose, control signals are issued from the logic bias board LBBto control the phase and attenuation module, PSAM.

In FIG. 4, a preferred embodiment of an antenna system according to theinvention has been shown comprising-three phased array antenna panelsA1, A2 and A3 with faces pointing out radially with an angle of 120°between each normal for the respective panels into respective sectors.The panels have the same number of antenna elements and are preferablybut not necessarily identical. As appears from the figure the panels areconnected via the AF and C networks to T/R modules located a distanceaway—however small it may be—from the panels and further on to mainunit, MU. The antenna elements of one given panel can also be arrangedin groups, which for instance are predisposed in given directions.

The antenna elements of a given panel number are connected to a givenport number of every transmit and receive unit. According to theinvention, each panel and corresponding sector is served one at a time,by operating the switches SM in each TRM module synchronously andselecting a given port number.

Hence, each T/R module serves all three panels through ports P1–P3 (n=3)in a sequential or time multiplexed manner, whereby during operationeach switch module SM of a particular T/R module serves a particularport number. Thereby, the antenna is beaming into each sector accordingto the desired beam pattern.

In base station applications, the sectors may be illuminatedsequentially with a fixed period and in a fixed order. For otherapplications such as radar, the panels may be served so as to givepreference to a desired direction or with a certain weight with regardto a certain sector, that is, serving a particular sector longer thanaverage according to the choice of an operator.

It appears, comparing the above antenna with a full permanent coverage,that the number of T/R modules, the feed and receive network complexity,and the exciter receiver unit processing power requirement are cut bytwo thirds.

In the above example, three panels are utilised. However, the inventiveconcept may just as well be applied to two or four panels or a largernumber of panels. If only two panels are used, the antenna panels can bemounted back to back, which provides for reduced dimensions of thecalibration and antenna feed networks.

Advantageous embodiments combining aspects of the first and secondembodiments shall now be described. FIG. 5–7 are schematic illustrationsof how couplers belonging to specific calibration nets, C1–Cj, aredistributed over an antenna panel. These panels may be plane rectangularpanels having antenna elements Dnhv, where n could denote the panelnumber (i.e. A1–A3), h could denote the horizontal position of theantenna element on a given panel number and v could denote the verticalposition of the antenna element on a given panel number.

In FIG. 5, a first embodiment is disclosed for arranging the calibrationnetwork, having the basic outline as shown in FIGS. 2–4. Threecalibration networks are provided, hence j=3 in FIG. 2 and three panelsare provided, n=3. The calibration networks are formed as separate nets(C1, C2 . . . Cj) not being connected to one another, each net beingconnected to a respective port (X1, X2 . . . Xj) on the calibration unit(CU). Each calibration network is dedicated to one panel exclusively;that is, all couplers of a particular calibration network are mountedadjacent antenna elements of the same antenna panel. As stated above,the calibration nets (C1–Cj) are independent with a minimum of mutualcoupling. Advantageously, the calibration nets (C1–Cj) are notduplicates in order to prevent the same error characteristics overtemperature from occurring.

According to a second embodiment for arranging the calibration nets,having the basic outline as shown in FIGS. 2–4, four differentcalibration networks, j=4, have been provided. The antenna panels havebeen shown in FIG. 6, in which the couplers of the respectivecalibration nets C1, C2, C3 and C4 are mapped to the antenna elements,Dhv, which arranged in a two dimensional plane at respective horizontaland vertical locations. Here, four couplers corresponding to calibrationnet C2 is arranged adjacent antenna elements D11, D12, D21, and D22 of aparticular antenna panel. Couplers connected to a calibration net C1 isarranged adjacent antenna elements D31, D41, D32 and D41. It appearsfrom the figure that calibration net C3 is associated with the fourlower left antenna elements and C4 is associated with the four lowerright antenna elements. As in the examples above, in total, for instancethree panels may be arranged, whereby the four calibration nets above,C1–C4 are arranged in the same manner for all three panels as shown inFIG. 5. Moreover, the layout of the couplers may be identical for thethree panels.

As mentioned above, the calibration nets are advantageously formed asseparate nets, which are isolated from one another, each calibration netbeing connected to a respective port X1, X2, X3 . . . on the main unitMU. However, some part of the calibration net could also be common andvarious calibration nets could branch off from a switch, such thatindividual branches of the calibration nets could be disconnected fromone another. Various calibration nets could also branch off from a node.Generally, it is desired that individual branches can be exposed tocalibration signals such that alternative paths through the calibrationnet or alternative calibration nets can be compared.

A third embodiment for arranging the calibration nets having the basicoutline as shown in FIGS. 2–4 is shown in FIG. 7. This embodiment issimilar to the FIG. 6 embodiment, but the couplers of the particularcalibration nets are distributed over the antenna panel in such a mannerthat no two couplers of the same calibration net is arranged adjacentone another.

For the above three embodiments, it should be understood that in mostpractical circumstances the number of antenna elements would be muchlarger, for instance thousands of antenna elements per panel.

The FIG. 5 embodiment above has the advantage that it requires a simplecalibration feed microwave transmission layout.

The FIG. 7 embodiment has the advantage that dimensional changes relatedto local areas of a given panel can be detected according to thecalibration routines according to the invention, as shall be explainedin the following. This is particular advantageous for applications wherethe panels are subject to harsh climate changes leading to local hot andcool spots. These phenomena typically occurs for air cooled air borneradars.

A first calibration routine of the invention shall now be explained withreference to FIGS. 8, 9 and 10. This routine could relate to any of theembodiments described with relation to FIGS. 2–6, described above.

According to FIG. 8, a calibration signal is output on port X1 andtransferred on calibration net C1. A signal is derived via the couplerQ111 associated with antenna element D111. The signal is transferredthrough respective antenna feed AF111 by operating the switch to P1 inTRM1. All other ports of all other T/M modules are closed.

The attenuation and phase delay CS_(111R) of the signal is measured.This value is compared with fixed values CS_(111Rfix) derived forinstance from a reference measurement using additional calibrationapparatus. The result of the comparison, Δ111, corresponds to theattenuation and phase delay differences at a given time in relation tothe reference values in T/R module 1, antenna feed AF111 and thecorresponding branch in receive and transmit network R.

The set of values Δ111 is stored for being able to compensate theamplitude and phase of signals from or to the exciter and receivermodule for accomplishing the directional steering capabilities of theantenna system. This corresponds to the conventional calibration of thesteerable antenna system.

Subsequently, a new calibration signal is emitted on port X2, as shownin FIG. 9, and transferred on net C2. The signal derived from thecoupler associated with element D211 is lead through port P2 of TRM1 andpropagated on the transmission network R into port XR and a delayCS_(211R) is measured.

CS_(211R) is compared with a fixed value CS_(211Rfix). The result, Δ211,as above, corresponds to the attenuation and phase delay in T/R module1, the antenna feed AF2 and the corresponding branch in receive andtransmit network R.

Besides errors in the port switch (1-n) in TRM1 together with thecabling of AF1 and AF2, the results from the two measurements Δ111 andΔ211 should be of the same size. If not, a drift in one (or both) of thecalibration nets 1 and 2 has been detected (or a drift in AF1 or AF2).

Hence, it can be established whether the system is functioning correctlyor not.

Subsequently, a new calibration signal is emitted on port X2 andtransferred on net C2, as shown in FIG. 10. The signal derived from thecoupler associated with element D2hv is lead through port P2 of TRMk andpropagated on the transmission network R into port XR and a delayCS_(2hvR) is measured.

CS_(2hvR) is compared with a fixed value CS_(2hVRfix). The result Δhv,as above, corresponds to the amplification and phase delay in T/R moduleTRMk, antenna feed AF2hv and the corresponding branch in receive andtransmit network R.

This routine is repeated for all antenna elements and all calibrationnets.

Now we have a number of m times n measurements that should agree. As anexample, where n=3 and a situation is occurring where two measurementsagree, the third measurement and corresponding calibration net is mostprobably drifting.

Hence, apart from establishing whether the system is functioningcorrectly or not, in the latter case K can moreover be established inwhich calibration net there is a drift. Moreover, the driftingcalibration net can be calibrated by appropriate adjustment of the givenamplitude and phase settings for the individual T/R modules, allcontrolled over the databus XD of the excite and receive unit, ERU.

Hence, not only are the TIM modules and antenna feed examined withregard to drift, also the calibration nets are subject to an examinationfor drift. Thereby, the erroneous calibration net can be identified andcorresponding compensation can be carried out. This procedure can bedone virtually while the system operates or causing only a shortinterruption.

In FIG. 11, a third embodiment of the radar system according to theinvention has been shown. In this embodiment a number of couplers Cjhave been provided which are not associated with any antenna element.However, respective antenna feed lines AFj11, . . . AFjhv are providedto the respective couplers Cj for each T/R module. These antenna feedscomprises a dump impedance Dmp in order to match the antenna feed to theantenna feeds connecting to the dipole elements. Hence, according to theinvention a calibration of the calibration net is also rendered possiblefor systems in which n=3 (TRM ports) or systems which does not utilisetime multiplexing between different sectors, n=2 (TRM ports).

In FIG. 12, the receive transmission network (R) is replaced with adigital transmit and receive transmission network, whereby A/Dconverters are arranged in or integrated with each TRM. Appropriatedigital signalling are transferred over a databus XD from the transmitand receive unit ERU. In FIG. 13, the arrangement of the A/D converterin connection to a T/R module has been shown. As will be understood thecalibration properties explained above also pertains to this embodiment.

1. An active antenna system (AS), said system comprising: a plurality ofantenna elements (D); an exciter/receiver unit (ERU); a plurality oftransmit/receive (T/R) modules (TRM1–TRMk); a feed and receivetransmission network (R) connecting said ERU and said plurality of T/Rmodules; a plurality of antenna feeds (AF) connecting said T/R modulesand said plurality of antenna elements; a beamforming unit (BFU),wherein said beamforming unit (BFU) controls the phase and amplitude ofsignals transmitted through each T/R module to produce a desiredbeamform from said plurality of antenna elements; a calibration unit(CU); a calibration network coupling said calibration unit (CU) andpoints on said antenna feeds (AF), wherein said calibration networkcomprises a plurality of alternative calibration nets (C1, C2 . . . Cj),and wherein each T/R module comprises a switch (SM) for switchingbetween alternative antenna feeds (A/F) and thereby between alternativecalibration nets.
 2. The active antenna system according to claim 1,wherein calibration signals from alternative calibration nets (C1, C2 .. . Cj) are selectively compared to determine whether a drift in atleast one of said calibration nets has occurred.
 3. The activeantenna-system according to claim 2, wherein each of said alternativecalibration nets (C1, C2 . . . Cj) is coupled to a plurality of saidantenna elements (D) by means of a plurality of signal couplers (Q). 4.The active antenna system according to claim 3, wherein each of saidplurality of signal couplers (Q) is coupled to one of said plurality ofantenna feeds (AF).
 5. The active antenna system according to claim 2,wherein at least one of said plurality of calibration nets (C) iscoupled to an antenna feed (AF) not associated with an antenna element(D).
 6. The active antenna system according to claim 1, wherein ones ofsaid plurality of antenna elements (D) are arranged in a plurality ofgroups, such as panels, being predisposed in a certain direction inrelation to one another.
 7. The active antenna system according to claim6, wherein each of said plurality of calibration nets (C) serves aspecific one of said plurality of groups.
 8. The active antenna systemaccording to claim 6, wherein each of said plurality of calibration nets(C) serves a plurality of antenna elements (D) from different ones ofsaid plurality of groups.
 9. The active antenna system according toclaim 6, wherein each of said plurality of calibration nets (C) serves aplurality of antenna elements (D) from different ones of said pluralityof groups, but no calibration net serves adjacent antenna elementswithin the same group.
 10. The active antenna system according to claim1, wherein each of said plurality of calibration nets is separate, or atleast may be disconnected, from one another.
 11. A method of operatingan active antenna system having a plurality of antenna elements (D); anexciter/receiver unit (ERU); a plurality of transmit/receive (T/R)modules (TRM1–TRMk); a feed and receive transmission network (R)connecting said ERU and said plurality of T/R modules; a plurality ofantenna feeds (AF) connecting said T/R modules and said plurality ofantenna elements; a beamforming unit (BFU), wherein said beamformingunit (BFU) controls the phase and amplitude of signals transmittedthrough each T/R module to produce a desired beamform from saidplurality of antenna elements; a calibration unit (CU); and acalibration network coupling said calibration unit (CU) and points onsaid antenna feeds (AF), wherein said calibration network comprises aplurality of alternative calibration nets (C1, C2 . . . Cj), and whereineach T/R module comprises a switch (SM) for switching betweenalternative antenna feeds (A/F) and thereby between alternativecalibration nets, said method comprising the steps of: coupling a firstcalibration signal from a first of said plurality of calibration nets toa first subset of said plurality of antenna elements, said firstcalibration signal being transferred through a first antenna feed (AF)by operating a switch in one of said plurality of T/R modules (TRM1)such that the signal is passed through a first branch in said transmitand receive network (R) to a port (XR) of said exciter/receiver unit(ERU), all other switches in other ones of said plurality of T/R modulesbeing operated such that no other signal is transferred through thetransmit and receive network to said port (XR); measuring theattenuation and/or phase delay (CS111R) of the first calibration signaland storing as a first result (Δ111); coupling a second calibrationsignal from a second of said plurality of calibration nets to a secondsubset of said plurality of antenna elements, said second calibrationsignal being transferred through a second antenna feed (AF) by operatinga switch (SM) in the same T/R module (TRM1) such that the secondcalibration signal is passed through the same first branch in saidtransmit and receive network (R) to said port (XR), all other switchesin other ones of said plurality of T/R modules being operated such thatno other signal is transferred through the transmit and receive networkto said port (XR); measuring the attenuation and/or phase delay (CS211R)of the second calibration signal and storing as a second result (Δ211);and if the two results (Δ111 and Δ211) are different, establishing thata drift in at least one of said first and second calibration nets (C)has occurred.
 12. The method of operating an active antenna systemaccording to claim 11, further comprising the steps of: coupling a thirdcalibration signal from a third of said plurality of calibration nets toa third subset of said plurality of antenna elements, said thirdcalibration signal being transferred through a third antenna feed (AF)by operating a switch (SM) in the same T/R module (TRM1) such that thethird calibration signal is passed through the same first branch in saidtransmit and receive network (R) to said port (XR), all other switchesin other ones of said plurality of T/R modules being operated such thatno other signal is transferred through the transmit and receive networkto said port (XR); measuring the attenuation and/or phase delay (CS311R)of the third calibration signal and storing as a third result (Δ311);and if two results through the same T/R module correspond to one anotherwhile a third result through the same T/R module differs from theothers, establishing that a drift in the latter calibration net hasoccurred.
 13. The method of operating an active antenna system accordingto claim 12, wherein at least the first, second or third result (Δ111,Δ211, Δ311) are stored for compensating the amplitude and/or, phase ofsignals from or to the exciter/receiver unit (ERU) for accomplishing thedesired directional steering capabilities of said active antenna system.14. The method of operating an active antenna system according to claim12, further comprising the step of calibrating a drifting calibrationnet according to the calibration nets that yield corresponding results.15. The method of operating an active antenna system according to claim11, further comprising the step of repeating the method for all antennaelements and all calibration nets.